I think vocalization is maybe a small part social roles and a big part because of what you're actually doing during sex. It's nothing universal, but in a heterosexual relationship, often, the man will be doing most of the moving. Of course, this point is probably partly because of social roles as well, but the simple fact remains that when you're moving, you feel less inclined to talk (or vocally communicate in another way).
Also, the desire to express your pleasure/love/whatever to your partner can be channelled through your movements, somewhat, when you're the one doing the moving. When you have a more passive role in the sexual act, well, the voice is probably a better option.
Same goes with masturbating. The typical "male" masturbation is rather intense physically, which leaves the mouth busy breathing. Your typical female, however, is often going to burn less calories in her arm when masturbating.
That, and in feelings and everything else, females tend to be more expressive and, well, vocal, period. Surely that's a rather important factor in sex. Men tend to want to show their feelings more than say them, and vice versa for women.
Personally, uhm, sex, I don't do such things, but even though how often I masturbate has drastically reduced with HRT, I do think I've been moaning more than I used to, somewhat.
